In Canadian Pat. No. 1,064,056, Reber et al describe a total isomerization process wherein large fluctuations in the concentration of either n-pentane or n-hexane in the reactor feed are prevented by suitably controlling the operation of a three-bed adsorber system. According to the disclosure, no more than two beds are being desorbed at any given time and the terminal stage of desorption in one of the three beds is contemporaneous with the initial stage of desorption in another of the three beds.
Both adsorber-lead and reactor-lead processes are then specifically exemplified. The adsorber-lead process calls for first passing a combined feed comprised of fresh feed and the total reactor effluent through the adsorbers to remove non-normal hydrocarbons so that the feed to the reactor is essentially normal hydrocarbons. This requires adsorbers of significant size and large recycle inventories which, in turn, require larger reactor volumes.
In U.S. Pat. No. 4,210,771, Holcombe describes a reactor-lead total isomerization process which reduces the recycle rate to the reactor while still maintaining a sufficient hydrogen partial pressure to protect the catalyst against coking. The partial pressure of the hydrogen in the reactor is a function of the hydrogen concentration. The system maintains the flow rates of hydrogen in the recycle and the combined reactor feed at constant levels, and varies the flow rate of fresh feed in an inverse relationship with desorbed normals in the recycle. By thus eliminating fluctuations in hydrocarbon flow rates to the reactor, the recycle flow rate to the reactor is reduced without risking an insufficient partial pressure of hydrogen to protect the catalyst. However, this reactor-lead process required recycle of all normal hydrocarbons, and the reactor, adsorber and recycle system capacities sufficient in size to accommodate this.
It would be desirable to reduce recycle flow rates and operate smaller reactors and adsorbers, and to otherwise improve operating efficiencies based on constant throughputs while still significantly increasing the octane of the feed hydrocarbon.